Various protocols, such as the Network Time Protocol (NTP) standard, which transmit timings using time stamps, have been developed. In these protocols, each slave device generally uses a local free-running clock signal, and tries to synchronize the local free-running clock signal with a clock signal of a master device (for example, an IEEE 1588 grand master) when the slave device is connected to the master device.
In general, when the slave device tries to synchronize with the master device, the slave device transmits to the master device a transmission message including a time stamp T1 regarding a transmission time. The master device receives the transmission message and transmits a response message to the slave device. This message transmission method may be referred to as a two-way message scheme. The response message includes a time stamp T2 regarding a time at which the master device has received the transmission message, and a time stamp T3 regarding a time at which the master device has sent the response message. A time stamp T4 regarding a time at which the slave device receives the response message also is used for synchronization. The slave device measures propagation delay using the time stamps T1 through T4, and synchronizes with the master device by subtracting the propagation delay from T3. In IEEE 1588, separate messages may be used for synchronization and propagation delay measurement.
Where network delay is symmetrical, delay in a downlink may equal to delay in an uplink, and accordingly, propagation delay and phase offset between the slave device and the master device may be obtained by the following Equations 1 and 2, respectively.
                              propagation          ⁢                                          ⁢          delay                =                                            (                                                T                  ⁢                                                                          ⁢                  4                                -                                  T                  ⁢                                                                          ⁢                  3                                            )                        +                          (                                                T                  ⁢                                                                          ⁢                  2                                -                                  T                  ⁢                                                                          ⁢                  1                                            )                                2                                    (        1        )                                          offset          ⁢                                          ⁢          from          ⁢                                          ⁢          master                =                                            T              ⁢                                                          ⁢              4                        -                          T              ⁢                                                          ⁢              3                        -                          (                              propagation                ⁢                                                                  ⁢                delay                            )                                =                                                    (                                                      T                    ⁢                                                                                  ⁢                    4                                    -                                      T                    ⁢                                                                                  ⁢                    3                                                  )                            -                              (                                                      T                    ⁢                                                                                  ⁢                    2                                    -                                      T                    ⁢                                                                                  ⁢                    1                                                  )                                      2                                              (        2        )            
Here, propagation delay calculated by the Equation 1 is an average value of delay times in both the downlink and uplink.
However, in an actual network, since network delay is asymmetrical, actual propagation delay in an uplink may be different from actual propagation delay in a downlink. Accordingly, in the case of an asymmetrical network, an offset value calculated by the Equation 2 between a master device and a slave device may be different from an actual offset value.
Where a master device is directly connected to a slave device not via any internode, propagation delay may correspond to a sum of a delay time by a transmission physical layer (PHY), a delay time by a cable, and a delay time by a reception physical layer (PHY). Since the delay times may be very stable and a time offset between the master device and the slave device appears as static time errors, the delay times may not increase delay variation values, such as jitter or wander.
However, in an actual network, where a slave device is connected to a master device via at least one bridge or routers that cannot process timestamp messages, time stamp messages that are transmitted by the master or slave device will be subjected to queuing delay which may randomly change over time, that is, according to traffic loads or traffic characteristics. In this case, the queuing delay may vary by several ms, which may not satisfy the requirements regarding delay variation, such as jitter or wander, which may be required by general applications, such as timing control of a wireless base station, transmission of non-compressed digital video and audio, etc.
While messages including time stamps at an internode of a network may be transmitted first, since bridges and routers generally use a priority mechanism based on a non-preemptive scheduling scheme, the messages that contain the timestamps may be delayed due to packets that are currently being serviced. As a result, in this case, delay variation may still occur in units of hundreds of μs.